My wife said she wanted one as she posted our daughter into hers. I managed to find her a stunning bright yellow fleece onesie which she got for her birthday from our daughter. Wen she put it on there was 10 minutes of raucous laughter when we found the arseflap (tm) - it had a zipped bum flap for, we assume, going for a dump or being taken from behind, perhaps. Anyway, cough, it's special. And she lost a bet and had to wear it to he supermarket. Of course she met loads of people she knew.

i'm sure they're brilliant if you're still in nappies.
once you can dress yourself, i say they're daft.
edit: especially as they need to build in dump flaps. didn't anyone learn the mistake of 1980's onesies ski suits? Need to get completely undressed just for a wee... and have you seen the state of a french bog in a ski resort?